In all shared communication systems having bandwidth limitations, there is a need for time synchronization to efficiently use the bandwidth resources that are available on the system. The nodes within the communication system need to be synchronized to each other to determine the boundaries of their allocated transmission slot, as determined by the media access scheme being utilized. For example, mobile units within a wireless communication network need to be synchronized with a base-station to aid the mobile units in the channel access method being used. Such channel access methods may include Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), demand-assignment schemes (DA/TDMA, DA/CDMA, DA/FDMA), priority-oriented access, or any other methods of allocating access to a data stream on a shared bandwidth. For 3rd and 4th Generation cellular communication systems, the multiple-access schemes may include Enhanced Data-rates for GSM Evolution (EDGE) and Wideband Code Division Multiple Access (WCMDA).
One manner of synchronizing nodes within a network includes having all the nodes on a common clock. For example, if TDMA is used for channel access in wireless communications, the mobile units need to be synchronized to a common clock to determine what the time-slot boundaries are and what are the allocated time slots for the mobile unit. In a further example, if demand assignment is used for channel access, the mobile units need to be synchronized to a common clock so that the base-station can assign them an appropriate schedule for transmission. Virtually all other types of media access schemes require some form of synchronization in order to communicate with other nodes, such as between computers in a network or between a base-station and mobile units in a cellular network.
Most wireless systems and other communication systems synchronize the nodes by using preambles. Preambles are a special pattern of bits that are transmitted at the beginning of a transmission (e.g., during the registration process) as part of the physical layer. These preambles are recognized by the nodes causing them to synchronize their internal clocks to the same time. However, the use of preambles requires special purpose hardware in order to recognize the special bit pattern and synchronize the transmission. The cost of such specialty hardware is generally greater than the cost of “over-the-counter” chips available on the open market and in mass quantity. Thus the cost of the devices in which these chips are implemented and which use preambles for synchronization is increased. Furthermore, this method of synchronization requires the receiving node to be synchronized at the beginning of the transmission, since the preamble is not repeated elsewhere during the transmission. If the node loses synchronization during the transmission or fails to read the preamble at the beginning of the transmission, then the node is unable to synchronize or re-synchronize and may end up losing the connection.
Another manner of synchronizing nodes in a network includes transmitting timing information within the message. However, adding timing information to the original message increases the amount of data that needs to be transmitted. For example, a message may be transmitted in a series of data packets. Each data packet may include a header and a packet payload. The header provides various instructions on how to read the message. The packet payload carries the content that is being transmitted from one node to another. A time stamp may be added by the transmitting node to the packet payload which is then read by the receiving node. The receiving node may then adjust its internal clock to the time indicated by the time stamp and synchronize with the transmitting node. However, placing a time stamp in the packet payload leaves less room for the content of the transmission. Thus, it takes longer for the transmission to be completed and makes the connection less efficient. Other synchronization schemes, such as including timing information in a broadcast as used by the Multimedia Cable Network Systems (MCNS) group, may be somewhat complicated thereby requiring more computing power or specialized chips.
Furthermore, it is expected that 3rd and 4th generation mobile communication networks and broadband wireless systems will communicate using speeds of between a few hundred kilobits per second to a few megabits per second. This allows high-speed transmission of various media types that are generally embodied in large data files, such as high-resolution photos, real-time videos, music, etc. This means a vast increase in the amount of data that may be sent over a wireless system. 3rd and 4th generation systems will also need to allocate resources among a limited bandwidth at a greater efficiency given the increase in the amount of data that is being transmitted.
In addition, many computers communicate over a broadband communication system using, for example, cable modems, Ethernet, digital subscriber line (DSL), etc. However, the bandwidth resources of a broadband communication system for computers are also limited, and a media access scheme needs to be used to allocate those resources efficiently. In order to implement the media access scheme for the computers, the computers need to be synchronized with a server or another system that allocates the resources. Much of the information being transmitted is done using the MPEG transmission format, so many of the modems that are used are also implemented with MPEG standard hardware. However, synchronization is often done according to the methods described above, thereby creating the same disadvantages.
Currently, it is expected that these transmissions will use the MPEG-2 transmission format. For example, both the European standard, Digital Video Broadcast (DVB), and the American standard for wireless video transmission uses MPEG-2 frames for transporting video information. MPEG is also applicable in point-to-multipoint wireless system operating in the Multi-channel Multi-point Distribution Service (MMDS) or Local Multi-point Distribution Service (LMDS) frequency bands. In addition, MPEG is applicable to broadband cable, optical fiber systems, or other high-speed hard-wire systems. Thus, many communication devices may be implemented with MPEG standard hardware for (de)multiplexing MPEG frames, thereby reducing the costs of these chips as they become more widespread. In addition, 4th generation systems may be designed to use the modulation and coding schemes used by DVB (i.e., ITU J.83 Annex A/Annex B specifications). Many cable modem products also use the ITU J.83 Annex A specification for the physical layer.
Thus there is a need for a method and apparatus that can implement time synchronization among multiple nodes in a communication system and reuse the transmission hardware within the node. There is also a need for a method and apparatus that can provide a common clock for time synchronization without requiring further information to be added to the data stream which sacrifices space for the content.